1. Field of the Invention
This invention relates to the manufacture of ceramic substrates, such as multilayer ceramic ("MLC") substrates and, more particularly, to the process of manufacturing MLC substrates having input-output pad surface metallurgy with increased resistance to structural failure.
2. Description of Related Art
In order to satisfy the increasing need for higher performance packaging of integrated circuits, MLC substrates are being developed with higher density input-output ("I/O") pad structures. However, the use of new materials to build the required I/O pad structures as well as changes in the processing of ceramic substrates after sintering, has increased the incidence of substrate mechanical failures at the metal-ceramic interface, such as ceramic tear out ("CTO") of the I/O pads.
Generally, a weak I/O pad structure is one which fails when a low pulling force is applied to the I/O pad during I/O pad strength testing. A particular case known as "low force" structural failure is understood as failures of the I/O pad when a pull force of less than 10 pounds is applied in a pin-pull test. In a typical Alumina MLC package where, for example, 21-pound pins are used, the I/O pad is expected to withstand 21 pounds of force applied to the I/O pin in a pin-pull test without structural damage. If a force in excess of 21 pounds is applied to the I/O pin it is expected that shank failure of the I/O pin will occur. In this example, a weak I/O pad is one which structurally fails when a force of less than 21 pounds is applied to the I/O pin.
Certain of the mechanisms underlying the low force failures of I/O pad structures are known, and include pin-shank or solder ball failure, ceramic tear out, planar failures at the metal-ceramic interface and braze failure. In the case of high strength CTO, the I/O pin pulls out a large piece of the ceramic structure with it, the piece typically having a diameter larger than a third of the size of the I/O pad diameter. In Alumina substrates built with 21-pound pins, this type of failure occurs for a pin-pull force typically above 10 pounds and is directly dependent on the brazing material volume located in the proximity of the I/O pad perimeter. Lower amounts of brazing material near the I/O pad perimeter, increased pin centrality within the I/O pad, and optimized brazing material volume near the center of the I/O pad, typically minimizes this type of failure. A ceramic braze dam is sometimes used to control this type of problem. In the case of a low strength CTO pin-pull failure, the I/O pin pulls out a small piece of the ceramic, typically less than a third of the size of the I/O pad. On an Alumina substrate built with 21-pound pins, a low strength CTO pin-pull failure typically occurs for a pin-pull force less than 10 pounds. An extreme case of failure occurs when most of the fracture line travels along the metal-ceramic interface and little or none of the ceramic is pulled out with the I/O pin. This case, also classified as planar or interface failure, occurs less frequently. On an Alumina substrate built with 21-pound pins, a planar failure can easily occur for pin-pull force below 5 pounds.
While there is a general understanding of the mechanisms underlying these failures, an unacceptable fraction of the low force failures occur in Alumina MLC's for unknown reasons. Methods to reduce the incidence of these low force metal-ceramic interface failures have included the use of a ceramic braze dam placed above the I/O pad after sintering to reduce the stress caused by the braze-pin structure at the edge of the I/O pad. However, the ceramic dam does not resolve the problem of the I/O pin failing at the center of the I/O pad.
It is also known to alter the composition and manufacture of ceramic green sheets to control the strength of the ceramic material. U.S. Pat. No. 5,045,402 discloses a method of producing a toughened glass ceramic through the use of Zirconia particles. While this process produces a green sheet capable of withstanding increased stresses on the substrate, it does not improve the metal-ceramic interface binding strength under an I/O pad.